Twin beam infrared absorption analyzer

ABSTRACT

A modulated twin beam selective radiation absorption monitoring analyzer. Beam modulation is converted to modulated electrical quantity fed to an amplifier and thence to a rectifier to give reading, the rectifier having a sensitivity threshold exceeding the amplified zero drift prior to rectification. Sample cell length is chosen such that the amount of absorption therein is in the range of 40 to 60 percent. The absorption by a sample of known composition is simulated by a change in beam intensity.

United States Patent 1 Schaefer June 19, 1973 [54] TWIN BEAM INFRAREDABSORPTION 2,951,939 9/1960 Luft 250/435 Z R 3,123,295 3/1964 Martin250/43.5 X ANALY E 3,430,041 2/1969 Kaye 250/435 [75] Inventor: WernerSchaefer, Kelkheim,Taunus,

Germany Primary Examiner-William F. Lindquist [73] Assignee: Hartmann &Braun Attorney-Ralf Siegemund Aktiengesellschaft, Frankfurt am Main,Germany 57 ABSTRACT [22] Fil d; J l 22, 1968 A modulated twin beamselective radiation absorption I monitoring analyzer. Beam modulation isconverted to p 746,618 modulated electrical quantity fed to an amplifierand thence to a rectifier to give reading, the rectifier having [52]U.S. Cl 250/435 R a i i y hr shold exceeding the amplified zero [51]Int. Cl. G0ln 21/34 drift prior to r ctification. Sample cell length ischosen [58] Field of Search 250/435 Such h he mo nt of absorptiontherein is in the range of 40 to 60 percent. The absorption by a sample[56] R f r Cit d of known composition is simulated by a change in beamUNITED STATES PATENTS 9/1955 l-leigl et al. 250/435 intensity.

3 Claims, 4 Drawing Figures Patented June 19, 1973 3,740,555

3 Shoots-Sheet 1 Fig. I

3 Shoots-Shoot 2 Fig.2

Vol /o CO Patented June 19, 1973 3,740,555

3 Sheets-Sheet 5 Vol 9'0 CO Fig.3

TWIN BEAM INFRARED ABSORPTION ANALYZER BACKGROUND OF THE INVENTION 1.Field of the invention A twin beam analyzer making use of selectiveabsorption with beam modulation electrically detected and resultingcurrent is rectified.

2. The Prior Art 1 There are well known selective twin-beam infra-redgas analyzers comprising a measuring and a reference beam, beammodulation and phase-independent rectification of the output voltage ofan amplifier associated with an electrically operated radiationdetector. The operating principle of such gas analyzers is well known.However, in such instruments it is desirable that the concentration ofthe measured gas be indicated by the associated measuring instrument ona substantially linear scale. Linearity is substantially achieved by sochoosing the length of the measuring cell that the selective absorptionof radiation by the measured component in the cell is relatively slightand does not ever exceed 40 percent. Up to this degree of absorption therelationship between the concentration of the measured gas in percent byvolume and the output signal, i.e., the current through the electricalindicating instrument, is approximately linear.

With reference to the measuring properties of this instrument it shouldbe observed that it is liable to instability of its zero point and ofits sensitivity due to optical asymmetries caused by technical factors.As a matter of experience the asymmetry which can be expressed by thequotient (A I)/I may be :t 0.1 percent per week where AI is theintensity difference between the two beams due to zero point drift and Iis the radiation intensity in the absence of the measured gas atcomplete zero balance. The measuring error due to zero point drift mayassume considerable proportions in course of time. For maintaining theinstrument it is therefore essential that the zero point as well as theend point of the measuring range should be checked daily and at leastweekly and the instrument rebalanced by adjusting means, test gasesbeing normally used for this purpose. The considerable amount of workthus involved in maintaining the instrument cannot be avoided in priorart instruments if the guaranteed accuracy is to be achievedthroughoutthe measuring range.

Now there are special applications of an infrared gas analyzer in whichthe instrument is principally used as a portable piece of equipment andwhere extreme measuring accuracy is not needed. These are for instancecases in which a given, relatively low, concentration limit is to bemonitored. An illustrative case is, for example, the supervision of thecontent of carbon monoxide in the exhaust gas of an internal combustionengine, where a maximum concentration of 4.5 percent by volume of CO isprescribed. A determination of concentrations in the direct vicinity ofzero or less than I percent by vol. of CO, and considerably beyond theupper limiting concentration, i.e. greater than 6 to 8 percent by volumeof CO, is not wanted, since such concentrations are of no interest. Inthis and similar cases a measuring accuracy of percent related to theend of the measuring range at 10 percent by volume of CO is quiteadequate.

The object of the present invention is the provision of a twin-beaminfrared gas analyzer of the specified type which is suitable for theselatter special applications. The problem is that of providing aninstrument of the greatest possible simplicity comprising the simplestpossible means for checking and readjusting this instrument with specialregard to the probability that the instrument will not be used byskilled personnel.

SUMMARY OF THE INVENTION According to the present invention there isprovided a selective twin-beam infrared gas-analyzer for monitoring gasconcentration comprising a measuring and a reference beam, modulation ofthe beams and phaseindependent rectification of the output voltageof anamplifier associated with an electrically operated radiation detector inwhich the length of the cell is so chosen that the selective absorptionin the measuring cell is between and 60 percent at the monitoredconcentration limit. The 'phase-independent rectification of the outputvoltage of the amplifier is effected in a rectifier network which has asensitivity threshold exceeding the amplifier output due to zero pointdrift prior to rectification; and at zero concentration the measuringand reference beams are so matched with respect to amplitude and phasethat the output current of the amplifier is zero.

In a preferred embodiment of the invention as employed, for example, formonitoring the presence of CO in the exhaust gas of internal combustionengines, the length of the measuring cell for a concentration limit of4.5 percent of CO is 10 mm and for other concentration limits of CO thelength of the cell in millimeters is given by the quotient45/Concentration limit.

For a check of instruments comprising a radiation source for both themeasuring and reference beams, a fixed resistor is shunted across thefilament radiating the measuring beam when the concentration of themeasured gas is zero, the resistance being so chosen that the resultingweakening of the radiant source corresponds to the attenuation the limitconcentration would cause, and the amplifier gain is adjustable. Theadjustment of gain is effected by a control knob on the instrument. Atthe same time as the fixed resistor is shunted across the filamentradiating the measured beam, a temperature-dependent resistor providedin the instrument is shunted across the indicating circuit, thereby tocompensate the effect of the ambient temperature on the absorptionexperienced by the measuring beam in the measuring cell.

The advantages afforded by these features of the present gas analyzerwill be understood as the description of an embodiment of an analyzerfor measuring the CO content of the exhaust gases of engines.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 is a diagram schematicallyrepresenting the optical and the electrical parts of the gas analyzer,

FIG. 2 is a graph illustrating the relationship between the percentageof the radiation absorbed by the measured gas in the measuring cell andthe concentration of the measured gas, on the one hand in the case of ananalyzer in which the absorption at the concentration limit is high andon the other hand in an analyzer in which this absorption is only onetenth of that in the first case,

FIG. 3 is a graph illustrating the relationship between theconcentration of the measured gas and the output signal of the analyzer,and

FIG. 4 illustrates the graduation of the scale of the indicatinginstrument of the analyzer.

DESCRIPTION OF THE PREFERRED EMBODIMENT With reference first to FIG. 1the analyzer contains two sources of radiation 1 and 2 each constitutedby an electrically heated filament fed from a commercial source of powerthrough constant current supply unit 3. After modulation by amotor-driven rotary shutter 4, the two beams first travel through ameasuring cell 5 and a reference cell 6 respectively and then enter thechambers of a detector which contains the gas that is to be measured. Inconventional manner, a diaphragm capacitor 8 in the detector, suppliedwith potential from a battery 9, generates a signal which is amplifiedin a controllable amplifier l and then applied through a rectifierbridge 1 l to an indicating instrument 12. The reference cell 6 containsa gas which does not absorb infrared radiation. The exhaust gas that isto be monitored flows through the measuring cell 5. The length of themeasuring cell is so chosen that when the concentration limit of 4.5% COin the gas has been reached the selective absorption of the beam willbe'50 percent. This is the case when the length of the cell is 10 mm.Under these conditions the absorption in the measuring cell within ameasuring range from 0 to 10 percent is represented by the curve 16 inFIG. 2. The curve considerably deviates from linearityyBy way ofcomparison a second curve 17 is shown in the graph which is thatobtained in an infrared gas analyzer calibrated to provide acharacteristic that is as nearly as possible linear. Linearity isobtained by reducing the selective absorption in the measuring cell atthe concentration limit of 4.5 percent by volume of CO to only percent.The illustrated relationships are due to the effects of Lambert-Beerslaw of absorption.

On the other hand, the relationship between absorption and output signalof the analyzer is linear. Thus the curve obtained for the output signalS as a function of the concentration of the measured gas is asreproduced by 18 in FIG. 3. Owing to the sensitivity threshold of therectifier the curve does not begin at zero. The sensitivity threshold isroughly at 4 percent of the measuring range and exceeds the zero pointdrift of the analyzer. The graduation of the scale of the indicatinginstrument in FIG. 4 results from the curve in FIG. 3.

By the choice of a percentage absorption of 50 percent, which isunconventional in gas analyzers for a mean value in the measuring range,zero point stability is decisively improved.

Compared with the absorption conditions at low per centage absorptionsas represented by the curve 17 in FIG. 2 which leads to a linearcalibration curve, zero point drift is reduced by one order ofmagnitude. In conjunction with the sensitivity threshold of therectifier this affords the advantage that the zero point of the analyzerwill remain stationary over long periods when a first calibration of theinstrument the measuring and reference beams at zero concentration havebeen so matched with respect to amplitude and phase that the outputcurrent of the amplifier is zero. Matching is done with the aid of meansconventional for such a purpose, such as by the interposition of stopsin the paths of the beams. The optical zero point is thus determined inprinciple. An extremely accessible adjusting means for zero pointcorrection is not required. If at zero concentration of the measured gaszero drift should nevertheless be observed to occur, then this wouldindicate that the instrument is faulty.

The non-linearity of the scale is quite acceptable. Nevertheless ameasuring accuracy of i 5 percent at the end point of the scale is stillguaranteed down to a measurement of 1 percent by volume of CO.

The high percentage absorption in the measuring cell provides theradiation detector with a relatively powerful signal. Contrary to thehigh gain amplifiers otherwise used the amplifier associated with theproposed analyzer need not have a high gain and may therefore besimpler. As will be understood from the following explanations the gainis adjustable.

For a functional check and for correcting sensitivity changes of theanalyzer a special circuit is provided. As illustrated in FIG. 1, thiscomprises a fixed resistor 13 and a temperature-dependent resistor 14. Amanually operable switch 15 permits a resistor 13 to be shunted acrossthe radiating filament of the radiation source 2 and at the same timethe resistor 14 to be connected in parallel to the indicating circuit ofthe indicating instrument 12. The resistance of 13 is so chosen thatwhen connected in the shunt the'resultant reduction in intensity of thebeam will simulate the intensity reduction that would be obtained at theconcentration limit. When both resistors are thus in circuit theindication given is checked. If the pointer of the indicating instrumentdoes not then exactly show a concentration of 4.5 percent by volume ofCO, the indication must be corrected by adjusting the gain of theamplifier 10 by turning a knob 19. In this simpke check andreadjustment, which can be quickly repeated as frequently as may bedesired, the purpose of the temperature-dependent resistor 14 is tosimulate the change in absorption of a gas containing 4.5 percent byvolume of CO in the measuring cell due to a change in temperature. Whenthis functional check has been performed the switch 15 is opened againand the instrument is then ready for measuring. The provision of thetemperature-dependent compensating resistor 14 does not mean that theanalyzer requires temperature compensation. When a measurement is beingmade there is no dependence of the indication upon the ambienttemperature, since changes in the different temperature-sensitivecomponents of the instrument in their totality are selfcompensating.

The infrared gas analyzer of the present invention therefore has onlytwo control elements, namely a switch for a functional check and acontrol knob for adjusting sensitivity.

With reference to a choice of a percentage absorption of 40 to 60percent at the concentration limit it may be observed that lowpercentage absorption cannot provide zero point stability for longerperiods when accurate measurements are required in the neighbor hood ofthe zero point and the measuring accuracy at higher percentageabsorptions is too low.

I claim:

1. In a selective twin beam infrared analyzer for monitoring gas for acomponent of maximum limited concentration, a pair of electric radiatorsconnected to a source of stabilized electric power for producing twinbeams, the analyzer having a zero drift tendency, and cells forcontaining reference and sample gas respec tively in said beams, meansfor modulating the beams, a detector for translating the modulated beamspassing through the cells to electrical voltage, an amplifier receivingthe voltage as input, a phase-independent rectifier for rectifying theoutput of the amplifier, and an electrical measuring instrumentconnected to the rectifier for indicating the output thereof as ameasure of 5 concentration of the component, the improvement comprisingthe sample cell being of a length such that absorption is between 40%and 60% at the maximum limit of component concentration, said amplifierhaving a variable gain control and rectifier having a sensitivitythreshold voltage greater than the output voltage of the amplifier dueto zero drift thereof and prior to rectification, and at zeroconcentration of the component, the measuring and reference beams beingso matched with respect to amplitude and phase that the output currentof the amplifier is zero; a fixed resistor, a switch for selectivelyshunting the resistor across the radiator for the measuring beam toreduce the intensity of same before passing through the sample cell, thethe resistance of the resistor being of the value to reduce theintensity of the measuring beam by the amount corresponding to theattenuation of the matched measuring beam passing through the samplecell when the latter contains sample gas at the maximum limit ofconcentration, for checking the functioning of the analyzer; means forvarying the gain of the amplifier; a temperature-dependent resistorselectively connected across the measuring instrument; and a switchganged with the first mentioned switch for shunting thetemperaturedependent resistor across the measuring instrument when themeasuring beam is reduced in intensity by closure of the first mentionedswitch, for compensating the effect of the ambient temperature on thepercentage absorption of the measuring beam in the sample cell.

2. In an analyzer as claimed in claim 1 for monitoring the CO content inexhaust gases of an internal combustion engine, the length of the samplecell in millimeters being the quotient 45/maximum concentration limit ofCO vol.

3. In an analyzer as claimed in claim 2, the maximum concentration of CObeing 4.5 percent.

1. In a selective twin beam infrared analyzer for monitoring gas for acomponent of maximum limited concentration, a pair of electric radiatorsconnected to a source of stabilized electric power for producing twinbeams, the analyzer having a zero drift tendency, and cells forcontaining reference and sample gas respectively in said beams, meansfor modulating the beams, a detector for translating the modulated beamspassing through the cells to electrical voltage, an amplifier receivingthe voltage as input, a phase-independent rectifier for rectifying theoutput of the amplifier, and an electrical measuring instrumentconnected to the rectifier for indicating the output thereof as ameasure of concentration of the component, the improvement comprisingthe sample cell being of a length such that absorption is between 40%and 60% at the maximum limit of component concentration, said amplifierhaving a variable gain control and rectifier having a sensitivitythreshold voltage greater than the output voltage of the amplifier dueto zero drift thereof and prior to rectification, and at zeroconcentration of the component, the measuring and reference beams beingso matched with respect to amplitude and phase that the output currentof the amplifier is zero; a fixed resistor, a switch for selectivelyshunting the resistor across the radiator for the measuring beam toreduce the intensity of same before passing through the sample cell, thethe resistance of the resistor being of the value to reduce theintensity of the measuring beam by the amount corresponding to theattenuation of the matched measuring beam passing through the sampLecell when the latter contains sample gas at the maximum limit ofconcentration, for checking the functioning of the analyzer; means forvarying the gain of the amplifier; a temperature-dependent resistorselectively connected across the measuring instrument; and a switchganged with the first mentioned switch for shunting the temperature-dependent resistor across the measuring instrument when the measuringbeam is reduced in intensity by closure of the first mentioned switch,for compensating the effect of the ambient temperature on the percentageabsorption of the measuring beam in the sample cell.
 2. In an analyzeras claimed in claim 1 for monitoring the CO content in exhaust gases ofan internal combustion engine, the length of the sample cell inmillimeters being the quotient 45/maximum concentration limit of CO %vol.
 3. In an analyzer as claimed in claim 2, the maximum concentrationof CO being 4.5 percent.